Method for producing spray-spun nonwoven sheets

ABSTRACT

SPRAY-SPUN NONWOVEN SHEETS HAVING IMPROVED PHYSICAL PROPERTIES ARE PRODUCED BY SPRAY SPINNING A FIBER-FORMING POLYMER TANGENTIALLY ONTO THE SURFACE OF A SHEET COLLECTION DEVICE. THE RANDOMNESS OF THE SPRAY-SPINNING PROCESS PROVIDES A UNIFORM SHEET HAVING BOTH LONG AND SHORT FILAMENTS.

Sept. 5, 1972 c. M. voer ETAL 3,689,342

METHOD FOR PRODUCING SPRAY-'SPUN NONWOVEN SHEETS Filed Dec. 8, 1970 FIG. 1

INVENTOR CLIFFORD MVOGT JOSEPH 6. POLISE ATTORNEY United States Patent US. Cl. 156167 3 Claims ABSTRACT OF THE DISCLOSURE Spray-spun nonwoven sheets having improved physical properties are produced by spray spinning a fiber-forming polymer tangentially onto the surface of a sheet collection device. The randomness of the spray-spinning process provides a uniform sheet having both long and short filaments.

BACKGROUND OF THE INVENTION This invention relates to spray-spun fibrous sheets and to a method and apparatus for producing continuous spray-spun sheets.

Various methods have previously been advanced for producing nonwoven fibrous materials and the like directly from extruded fiber-forming materials. In general, these methods form nonwoven materials by extruding a fiber-forming polymer in liquid (e.g. molten or plasticized) form through a plurality of orifices to form a like number of filaments which are either collected directly, or following an intermediate drawing stage, on a moving surface in the form of a mat. Examples of such methods are disclosed in US. Pats. 2,206,058; 2,382,290; and 2,810,426.

More recently, spray-spinning processes and apparatus have been developed which permit the formation of substantially continuous filaments at high production rates without the concurrent formation of shot and other undesirable physical forms, such as a predominance of very short fiber elements. More specifically, the spray-spinning nozzle which is best suited for use with organic thermoplastic fiber-forming polymers is designed to attenuate the fibe'rs while still in the plastic state, without excessive fiber breakage. Thus, the fiber is retained substantially as a continuous fiber or long-staple fiber. This is accomplished by attenuating the molten fiber exiting from the extrusion orifice with jets of gas at a relatively shallow angle of convergence wherein the gas jet tangentially contacts the extruded fiber. The angle of convergence is measured in a first plane containing the orifice axis and a projection of the passage axis. The first plane is defined by the axis of the extrusion orifice and by a line extending perpendicularly to the extrusion orifice axis and passing through the center of the outlet opening of one of the gas passages. The projection of the gas passage axis on the first plane intersects the orifice axis, the acute angle of intersection of the projected passage axis and the orifice axis of the nozzle is the convergence angle. This angle is a shallow angle converging toward the fiber axis. The convergence angle preferably ranges from about 3 to degrees and more preferably from about 4 to 7 degrees. However, as noted above, because the axis of the fiber extrusion and the axes of the gas streams are skewed with respect to each other, the respective axes do not actually intersect.

The skew angle is measured in a second plane that includes the orifice axis and is perpendicular to the first plane. The projection of the axis of the gas passage on the second plane intersects the orifice axis. The acute angle of intersection of these lines in the second plane is the skew angle. This angle is between about 1 and 10 degrees, and preferably between 1 and 7 degrees. Therefore, the fiber ice attenuation is effected by the close proximity of the gas axis to the fiber axis as they pass each other at their closest point. This closest point is called the convergence point.

The distance between the aforementioned axes at the convergence point is referred to as the convergence diameter. The convergence diameter ranges from about 0.5 to 18 millimeters and preferably from about 1 to 10 millimeters. The convergence point occurs at a distance of from about 12 to 125 millimeters from the fiber extrusion orifice. This distance can be varied in accordance with the convergence angle and the skew angle. It will be readily recognized by those skilled in the art that the various measurements noted above are interrelated and therefore the setting of certain angles and distances predetermines certain of the other angles and distances.

Various gas or jet pressures can be utilized in the present invention. For the gas jet dimensions described herein, pressures between about 10 and pounds per square inch gauge are normally used. However, higher or lower presures can be used, depending upon the particular desired operating conditions, the gas jet openings, the fiber being extruded and the like. Pressures of from about 20 to 25 pounds per square inch gauge have been found to be particularly desirable for organic fibers using the gas jet more specifically described herein.

Of course, in achieving optimum conditions for particular commercial operations, due regard must be had for the specific material and processing conditons. The inclinations selected for the non-intersecting axes of the generally converging gas jets may depend for example upon such factors as gas temperature, the velocity attained by the gas, the temperature and melt viscosity of the fiberforming matrial, and the deposition pattern at the zone of collection.

The aforementioned spray-spinning processes and apparatus have been used to produce spray-spun fibrous bodies comprising randomly arranged filaments having a varying degree of crystalline orientation and a varying filament diameter along their lengths. These nonwoven spray-spun fibrous materials eliminate or substantially lessen the need for various subsequent bonding treatment by random thermal and/or adhesive bonding. The filaments are bonded to each other at crossover points and the selfbonding gives the spray-spun structure substantial coherency, in addition to some degree of filament entanglement.

Nonwoven fibrous materials made by spray-spinning should have sufficient structural coherency and stability to retain their identity when handled manually or mechanically. The fibers o'r filaments themselves must not be made so resistant to stretching as to become brittle or susceptible to breaking when subjected to various operations to improve their properties for a particular end use. Nonwovens having insufficient tensile strength generally do not retain their dimensions on handlig due to their own weight. However, the spray-spinning operation may be controlled to obtain the desired amount of tensile strength by self-bonding of the tacky fibers to each other. A- bonding agent may also be utilized to increase or achieve the desired bonded strength to maintain structural coherency.

OBJECTS OF THE INVENTION It is an object of this invention to provide a novel method for producing continuous spray-spun fibrous sheets.

Another object of this invention is to provide a novel lengths ranging in length from long to short, randomly interentangled, to provide good strength and high structural coherency.

These and other objects will become apparent to those skilled in the art from a description of the invention.

SUMMARY OF THE INVENTION In accordance with the present invention, these and other objects of the invention are realized by spray-spinning substantially continuous filament material tangentially onto the surface of a collection device rotating in a direction countercurrent to the direction of filament spray. The collection surface is curved about the area of primary filament deposition so as to present a convex surface to the filament path and the collection device is preferably substantially cylindrical.

The spray-spun sheet produced has a large proportion of filaments which are spread out over relatively long distances before changing direction and, although having strong structural coherency, the sheet has fewer bond points, which leads to a softer sheet. The filament material itself has a varying amount of molecular orientation and is randomly bonded to itself at crossover points between filament sections during spray-spinning so as to form varying lengths subject to the drawing.

DESCRIPTION OF THE INVENTION The continuous spray-spun sheet is produced by sprayspinning tangentially a fiber-forming polymer onto the surface of a moving collection device positioned in front of one or more spray nozzles. The filaments from the spray nozzle are so directed at the collection device, as for example a horizontally positioned rotating cylindrical drum, that part of the time the filaments strike the base of the drum and at other times the filaments go past the base and pass partly around the drum. A new portion of the collection surface is moving constantly in a direction counter to the direction of the filament spray such that successive layers of filaments are deposited on the sheet during collection. Since the filaments are still plastic when they strike the surface of the collection device, some stick or bond points are formed at points where two or more filaments touch or cross each other.

A slightly curved smooth surface extends back tangentially from the base of the collection surface, e.g. a drum. This introduces a streamlining effect to the filaments as they pass beneath the drum. The extruded filaments are projected back in a substantially straight and uniform manner for considerable lengths along the smooth surface and are finally attracted to the smooth surface and adhere. Since the collection surface is continuously being rotated in a direction counter to the direction of filament spray, the fibers break free but move along the surface Without folding over on themselves and without fiopping around.

Prior to this invention, spray-spun sheets have been prepared by directing the filament from the spray nozzle perpendicularly toward the collection device, such that the filaments are randomly deposited on the surface. Using this prior method of collection, only short distances existed between bonding points between the various filaments. In addition, filaments did not extend for great distances along the length of the sheet. As a result, after the sheet had been drawn, it opened up, but the sheets were relatively stiff and lacked good drape characteristics. When torn, the stretched sheet tore by the breaking of only a few filaments at a time. In addition, little cooperation between filaments was found, which showed that there was not much of an interfiber network structure.

This invention can be used in connection with the production of nonwoven spray-spun sheets from any of the polymers which are melt or solution spinnable. Of the various fiber-forming polymers that can be extruded through a filament-forming orifice, the polyolefins, polyurethanes, polyesters and polyamides are preferred. Plaslticized cellulose esters (e.g. acetate and triacetate),

acrylics and polyacetal resins, especially polyoxymethylene copolymers, are examples of other polymers within the scope of this invention and are used with correspondingly good results. Suitable blends and copolymers are also within the scope of this invention.

Basically, the spray spinning is effected by extruding liquid fiber-forming material, which may be either molten, plasticized or dissolved in a solvent therefor, through an orifice as a filamentary material. Attenuation of the incompletely hardened filament is effected by a plurality of high velocity gas streams issuing from gas passages spaced about the extrusion orifice and having axes that converge toward, but do not intersect, the extrusion axis. The gas flow projects the filament away from the nozzle in an expanding conical pattern. The filamentary material projected from the spinning zone has desirable characteristics which have been found to be particularly beneficial in the construction of nonwovens. The fibers of this invention are in the form of a substantially continuous filament structure and exhibit a random arrangement of filaments having random variations in length and random lengthwise variations in diameter and degree of orientation which result from random variations in the attenuating action of the gas streams acting on the freshly spun filaments.

As the freshly spun filament is projected away from the spinning nozzle in an expanding conical pattern by the gas streams, the collection surface, i.e., the portion of the outer surface of a collection device, such as a drum, which is rotated countercurrently to the flow of filaments and moved continuously in the path of the projected filamentary material to collect the filamentary material without destroying the random distribution of the filamentary sections. The movement of the collector surfaces or of the spinning orifice serves to bring new sections of the surface into the path of the filamentary material continuously so that the collected fiber forms a fibrous nonwoven sheet that is continuously removed from the collection zone and the fiber is uniformly deposited. In a preferred embodiment, the rotating collection device may have a series of spaced projections, e.g. spikes or pins protruding from its surface. The pins act as collection points and in a large measure prevent the largely continuous spray spun fiber from flying too far past the collection surface. This also tends to keep more fiber in vertical alignment than if the pin projections were removed.

A curved surface extending tangentially back from the collection surface is utilized. Prior to this, the prepared sheet had filaments which extend up to about 12 inches in length back from the base of the collection device. Although the spinning process was completely random, the sheets had imperfections in them due to folding over and clumping of the loose filaments which extend back from e base of the collection surface, e.g. a drum. These fibers initially were rather loose and tended to flop and flutter about due to the expansion of the attenuating gas streams. With the present process, a curved surface extends back tangentially from the base of the take-up device and overcomes these shortcomings.

The surface is curved along its length in order to introduce a streamlining effect to the fibers as they pass by the collection device. With this streamlining effect (filament flow is substantially smooth, i.e., substantially no turbulence), fibers extend considerably longer distances back past the collection device, e.g. a drum (as much as about 30 inches) and finally strike and stick to the curved surface. Since the collection surface is rotating in a direction counter to the filament spray, the spun filaments, after their initial contact with the curved surface, are broken free and are drawn across the surface. In this manner, successive layers of filaments are deposited. The fibers, on being drawn across the curved surface, do not break free of the surface or flutter about. The resulting sheets do not show any imperfections and when spun sheet is biaxially stretched, a uniform open structure is produced.

The extrusion temperature, extrusion rate, speed of rotation and traverse of the collection device, and distance from the extrusion orifice to the collection surface are related to each other to assure that the collected sheets will have the desired properties and weight. While extrusion temperatures may be anywhere above the melting point of the polymer, it has been found that best results are obtained by heating the polymer to at least 150 degrees centigrade, and preferably from about 250 to about 350 degrees centigrade, above the softening point of the polymer being extruded. For example, polypropylene having hereinafter defined characteristics will generally be heated to temperatures of from about 325 to about 400 degrees centigrade. Polyethylene, on the other hand, will be heated to from about 350. to about 450 degrees centigrade. In the preferred embodiment, polymer 1s generally extruded at 1 to about 30' pounds per hour, and desirably at 2 to 15 pounds per hour.

The collection surface is ordinarily moving, e.g. rotating, at a speed sufficient to provide a moving surface of from about 25 to about 125 feet per minute. The speed of traverse f the collection surface or, optionally, the speed of traverse of the spray-spinning nozzle, is sufficient to provide a traversing surface of from about 5 to about 60 feet per minute, preferably from about 15 to about 50 feet per minute. In addition, multiple spray-spinning nozzles may be used to preclude the necessity of traversing either the nozzle or the collection device.

A single fiber generating source is used, the collection device is continuously rotated in a direction counter to the filament spray and the collection device is moved back and forth in a plane perpendicular to the direction of the advancing fiber. If several nozzles are used and filament streams overlap, the collection device need only rotate in a direction counter to the filament spray and there is little need for movement in a plane perpendicular to the direction of advancing fiber.

For best results, the portion of the surface of collection device first contacted should be from about 6 to about 48 inches, preferably 10 to 30 inches from the spray-spinning orifice. With greater distances, the spray pattern is difficult to control and the resultant web tends to be non-uniform. Shorter distances result in a web which contains shot, i.e., beads of non-attenuated polymer, which undesirably affects subsequent processing and web uniformity.

In the preferred nonwoven structure, the as-spun denier per filament can vary from about .05 to 60 (about 2.5 to 100 microns), with an average of from about 3 to 12. After stretching, the average denier per filament can be reduced to from about 0.25 to 3. In a nonwoven structure embodying this filamentary material, at least about 75 percent of the filaments have diameters from 1 to 50 microns. The smallest diameter segments represent a larger portion of surface area per unit volume, while the larger diameter segments are relatively stiff and resist crushing of the nonwoven.

These novel nonwoven sheets of this invention exhibit high strip tensile strength and good drapeability (in centimeters) to weight of the sheet (in milligrams per square centimeter) of from about 0.75:1 to :1, preferably less than 1.25:1. Drapeability is measured by determining the length of the nonwoven sheet which is necessary to cause the sheet to bend from the horizontal plane when under no constraint to such an extent as to contact a declining angle of 41.5 degrees of slope from the point of departure of contact. A strip of nonwoven spray-spun sheet 1 inch wide is placed upon a block of wood or other horizontal surface. Abutting the horizontal surface of this material is a 41.5 degree inclined plane, which at its top adjoins the horizontal surface. The tested specimen is placed with the narrow edge at the juncture at the horizontal and the inclined surfaces. It is then moved over the inclined surface until the free end touches the 41.5

degree slope of the testing block. The drapeability or drape stiffness, designated C, is measured in centimeters, being one-half of the free length of specimen extending beyond the horizontal surface edge. Another measurement, the cantilever stiffness or flexual rigidity (G) is determined by multiplying the weight (W) of the fabric [in milligrams per square centimeter times the cube of the drapeability in centimeters cubed (G=W C In the nonwoven sheet of this invention, filaments extend for great distances along the length of the sheet and greater distances exist between bond points (i.e., 2 or more filaments fused at the contact point) between the various filaments. This, in combination with greater cooperation between filaments, results in a nonwoven sheet having good drapeability and high tensile strength. From about 75 percent to percent of the filaments have lengths between bond points of from about 1 to 20 mils on the side not touching the collection surface (the air side). On the collection surface side, from about 75 percent to 90 percent of the filaments have lengths between bond points from about 1 to 40' mils.

In a SO-micron thick section of about 0.2 inch wide strips of the full thickness of the nonwoven sheet of this invention, there are present from about 50 to bond points, from about 10 percent to 25 percent unbonded filaments (an unbonded filament is a filament not bonded at a cross over point with another filament in the given section examined), and greater than 40 percent of the bonds, preferably from about 50 to 60 percent, consisting of from 2 to 4 filaments per bond point.

A better understanding of the invention may be had from a description of the drawings wherein:

FIG. 1 is a schematic drawing of the process of producing the spray-spun nonwoven sheets of this invention.

FIG. 2 is a schematic drawing of the process of this invention wherein the collection device has a series of spaced projections on its surface.

Referring to FIG. 1, a continuous spray-spun nonwoven fibrous sheet is produced by directing fibers spun from a fiber-generating spinneret 1 tangentially onto the bottom surface of a cylindrical collection device 3 and curved surface 4 extending back tangentially from the base of collection device 3 near the tangential point of fiber contact with the cylindrical collection surface. The collection device 3 is rotated countercurrently to the direction of the filament spray. The fibrous sheet 6 is continuously withdrawn between rolls 5 as the spray-spun fibrous sheet is formed. The smooth curved surface, which can be a metal plate, cardboard, etc., is represented by the line AFE, which is a substantially smooth arcuate curved downwardly disposed with respect to the collection device 3. The line DA is a line drawn down vertically from the curved plate (A') must be within about an inch of line center of the collection device. The front edge of the DA. The bottom point of the plate (point P) curves at a distance of from about 1 to 18 inches from the plate tip A nearest the collection device (i.e., the distance between points A and B ranges from about 1 to 18 inches, since the line BFC is parallel to the line DA). The line A'BE is a tangent to the circular collection device 3 and is perpendicular to line DA. The distance between points B and F can be varied from about 1 to about 3 inches. Point E represents the point where the smooth arcuate plate is again horizontal and distance AE can vary from about 10 inches to about 20 inches. The arcuate plate has a curvature which starts at a point G, which is from about 20 to 90 percent of the distance between the front edge of the plate A and the bottom point of the plate F (i.e., since the line HG is parallel to both lines AD and BFC, the line A'H is from about 20 to 90 percent of the line AB). The angle the arcuate plate slopes down initially from the collection device (i.e., the angle BAC) can range from about 5 degrees to 30 degrees, preferably from about 10 degrees to 25 degrees.

FIG. 2 is a schematic drawing showing the arcuate surface AFE and collection device 3 having projections 7 on its surface.

The use of a collection device having projections on its surface helps to prevent any sheet slippage during takeup. Other types of surfaces which work equally well include (1) an etched surface, (2) a porous, screened or foraminous surface through which a vacuum can be pulled to hold the sheet to the roll, or (3) a smooth roll with a nip roll attached at the top to help prevent sheet slippage. The projections on the collection device facilitate startup, since one can place a small piece of fibrous sheet as a leader on the projections and not worry about holding it in place until the newly formed sheet product comes. through. Once the system is started, the projections are unnecessary; they just make collection somewhat easier.

The following examples will serve to further illustrate the principles of this invention. All parts are by weight unless otherwise indicated.

Example I Polypropylene (Profax polypropylene type 6423) was extruded through a nozzle. The substantially continuous filament material formed was spray spun tangentially onto the bottom surface of a cylindrical drum having a diameter of about 6 inches and an arcuate metal plate extending back tangentially from the base of the drum. Referring to FIG. 1, the front edge of the plate (A') was one inch farther back away from the nozzle 1 than point DA; point P was about 8 inches back from the front edge of the plate (A'); point F was about 2 inches below point B, the distance A'E was about 15 inches; point G was about 3 inches from point A and angle BAC was about 20 degrees. The fibrous stream was deposited while the continuous filaments were in a molten or semi-molten state, to form a spray-spun, fibrous nonwoven sheet. The fibrous sheet, when out into a 12-inch finished length, weighed about 4 ounces per square yard or about 13.6 milligrams per square centimeter.

The polypropylene was spun at a temperature of 358 degrees centigrade using a 0.028 inch diameter nozzle, of the geometric configuration described in the aforementioned Wagner et al. application. Steam at 25 p.s.i.g. and 405 degrees centigrade was directed outwardly from the nozzle to surround the filaments issuing from the nozzle. The polypropylene was collected on a cylindrical drum positioned 11 inches from the nozzle so that the fibers tangentially strike the bottom of the drum allowing a high percentage of fibers to pass under the drum. The drum was rotating and taking up the spray-spun sheet at a speed of about 0.33 meter per minute, and the drum was traversed in the path of the projected filamentary material at a speed of 22 traverses per minute. A sample of this spray-spun sheet had an average denier per filament of about 7.25, varying from about 0.04 to 58.5 denier per filament. A 1 meter sample of this spray-spun sheet was then biaxially drawn at a draw ratio of 3.521 in the machine direction and 2:1 in the cross-machine direction to yield a nonwoven sheet having a weight of 0.70 ounce per square yard, or 2.4 milligrams per square centimeter.

The procedure outlined above was repeated except that the collection drum Was positioned so that the fibers struck the collection surface perpendicularly. The collection drum was rotated at a speed of .23 meter per minute and traversed at 29 traverses per minute respectively for 3 samples. These 3 samples were then drawn biaxially at various draw ratios to yield respectively nonwoven sheets having a weight of 0.372 ounce per square yard or .13 milligram per square centimeter, 1.09 ounces per square yard or 3.70 milligrams per square centimeter and 1.15 ounces per square yard or 3.89 milligrams per square centimeter.

The three perpendicularly collected sheets and the tangential sheet were then tested to determine the drapeability 8 according to the previously outlined procedure. Table I below shows the improved drape, stiffness and flexural rigidity of the tangentially collected nonwoven sheets of this invention.

TABLE I Weight Drape Flexural of sheet stiffness rigidity Collection type (mg/em?) (em.) (mg. cm.)

0) T T 1. 67 4. 77 5. 67 234 Perpendicular 3. 67 7. 49 7. 43 1, 563 1, 525 3. 84 5. 87 7. 34 819 1, 641 Tangential 3. 31 2. 71 2.05 47. 1 20. 4

1 Indicates direction of the sheet in relation to the direction at which the extruded filaments strike the collection surface.

Example 11 The procedure of Example I was followed to prepare seven perpendicularly collected spray-spun sheets and seven tangentially collected sheets. The spinning temperature, gas pressure and draw ratio were varied as outlined in Table II to give sheets having a strip tensile strength as shown in Table II below.

Table II shows that spray-spun sheets of much improved tensile strength are obtained by spray-spinning tangentially according to the process of this invention.

Spray-spun materials produced using various other polymeric fiber-forming materials such as polyamides, polyesters, cellulose acetates, polyurethane, acrylics, conjugates thereof and the like thermoplastic oplymers are used with correspondingly good results.

The fibrous structures of this invention may serve a variety of useful purposes. They may be used as filters and find particular use as cigarette filter material. With suitable coating and/ or laminations, they may serve in industrial applications instead of conventional woven materials, films and papers. The nonwoven structures of this invention can also serve in the preparation of felts, leather-like materials and suede-like materials. It may also be used as an interlining or interfacing material used in imparting shape to garments. In addition, the tubular nonwoven fabric may be used in the fabrication of clothing such as coats and work uniforms, or the like.

While various embodiments of the present invention have been described, the methods and elements described herein are not intended to limit the scope of this invention since changes therein are possible. It is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing the same results in substantially the same or equivalent manner. It is intended to cover the invention broadly in whatever form its principles may be utilized, being limited only by the following claims.

What we claim is:

1. A continuous process for making nonwoven fibrous structures comprising extruding a molten stream of synthetic, organic, melt spinnable thermoplastic polymer through a spinneret, tangentially onto the surface of a collection device, said collection device rotating in a direction countercurrent to the direction of filament spray, directing some of the filaments so as to strike the base of the collection device and other filaments passing around the surface of the collection device, streamlining said filaments that pass around the collection device and collecting said streamlined filaments on an arcuate surface which extends back tangentially from the base of the collection surface, and removing the nonwoven structure of randomly arranged filamentary material in the direction of rotation of said collection device.

2. The process of claim 1 wherein the melt spinnable thermoplastic polymer is extruded at from 250 degrees to 350 degrees centigrade above the softening point of the polymer, the collection surface is rotating at a speed suflicient to provide a rotating surface of from about 25 to 125 feet per minute, and the portion of the surface of the collection device first contacted by the filamentary material is from about 6 to about 48 inches from the extrusion spinneret.

3. The process of claim 1 wherein the collection device is rotated and simultaneously traversed back and forth in 10 a reciprocating manner in a plane perpendicular to the direction of the advancing fiber so as to provide a traversing surface of from about 5 to 60 feet per minute.

References Cited UNITED STATES PATENTS ROBERT F. BURNETT, Primary Examiner 15 R. O. LINKER, 1a., Assistant Examiner US. Cl. X.R. 

